Got a call 5 minutes ago stating that he found my lost credit card. When I asked the dude who he was, he told me I was harrassing him and hung up on me.
BLocking number can be done through the phone company. Some, but not all cell phones, have a feature to block a number, and some newer landline phones allow blocking a limited amount of numbers.
This isn't a generic question that can be answered as the method is dependent upon your phone/phone service.